1. Field of the Invention
The present invention relates generally to optical fiber-based sensors and more particularly to a fiber optic sensor for precision 3-D position measurement using mode coupling between single-mode fiber cores in a multi-core optical fiber.
2. Background Art
The idea of using multicore optical fibers for sensors has been known since the late 70's, when much of the seminal work on optical waveguide coupling was published. Meltz and Snitzer were granted patents (Meltz and Snitzer, 1981; Snitzer and Meltz, 1981) in 1981 for a fiber optic strain sensor and for a fiber optic hot spot detector which rely on the cross-talk phenomenon in multicore fibers. Their hot spot detector also features the innovation of multifrequency interrogation for localization along the fiber. However, only two or three frequencies were envisioned because their application was to locate a single hot spot of limited spatial extent. The major concerns expressed in their strain sensor patent were to increase the unambiguous range of response and to separate strain-induced response from temperature-induced response—the issue of localization was not discussed.
The most extensive body of work has been published in the relevant literature by R. Romaniuk and collaborators (Romaniuk 1990, 1985). His group fabricated and characterized multicore optical fibers with many different core geometries. Although the major focus of their research seems to be the use of multicore fibers for multiplexed communication, they presented some test results from four-core sensors used to measure bending strain, bending radius direction (with respect to core geometry), pressure, and temperature. Correlation between amplitude changes at the output cores and specific measured quantities was determined empirically rather than via models. One relevant result is their determination of a bending radius directional sensitivity of better than 1 degree. This measurement was made by winding a short (1 m) multicore fiber around a strain test drum and monitoring changes in relative output amplitude as the fiber was “rolled” down the drum under constant strain.
Several patents discuss various methods for producing multicore fibers (e.g., Watanabe et al. 1989).